Open hydraulic fluid flow circuit arrangement and method of controlling the hydraulic circuit

ABSTRACT

The invention relates to a fluid flow arrangement ( 1, 15 ) with an adjustable fluid pumping device ( 2 ) and a fluid working machine ( 12 ). The fluid working machine ( 12 ) is connected to the fluid pumping device ( 2 ) and a re-circulating loop is provided for the fluid working machine ( 12 ). The re-circulating loop is fluidly connecting a first fluid port (A) and a second fluid port (B) of the fluid working machine ( 12 ), where the first (A) and the second fluid port (B) are at times at a different pressure level. The re-circulating loop comprises a controllable fluid throttling device ( 21, 22 ), and a switchable fluid conduit device ( 23, 24 ), so that a defined decelerating force can be generated for the fluid working machine ( 12 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims foreign priority benefits under U.S.C. § 119 toGerman Patent Application No. DE102016122535.5 filed on Nov. 22, 2016,the content of which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The invention relates to a fluid flow arrangement that comprises anadjustable fluid pumping device and a fluid working machine that isfluidly connected to the fluid pumping device and that further comprisesa re-circulating loop, fluidly connecting a first fluid port and asecond fluid port of the fluid working machine, where said first andsaid second fluid port are at times at a different pressure level. Theinvention also relates to an electronic controlling device for such afluid flow arrangement (or a similar one).

BACKGROUND

Hydraulic transmissions for transferring mechanical energy from oneplace to the other (usually including the possibility to change somecharacteristics of the mechanical energy involved, like rotating speed,possible torque and the like) are used in several technical fields inthe meantime. An example for this is the field of wind generators,where, due to varying wind speed, an input force, coming from thepropeller and showing varying turning speed and/or driving torque has tobe transmitted to an electric generator. At the electric generator,however, usually a constant turning speed is required. Therefore, notonly the location where the mechanical torque is present is shifted, butalso some characteristics of the mechanical energy (for example theturning speed) are translated.

Another technical field for hydraulic circuits is used for propellingvehicles, in particular land vehicles. Although already some cars ortrucks are propelled using a hydraulic transmission their use is stillsomewhat limited, although some characteristics of hydraulic circuitsare promising. However, the use of hydraulic circuits for driving avehicle is the standard approach for special vehicles that are usinghydraulics for fulfilling their main duty. Since a hydraulic pump of aconsiderable size is thus present already, it is quite common to usehydraulic energy for propelling the respective vehicle as some kind of aspin-off as well. Examples for such machines are forklift trucks,excavators, shovel dozers and the like.

From a technical viewpoint, closed hydraulic fluid flow circuits arepreferred for propelling a vehicle due to their intrinsic properties. Onthe other hand, open fluid flow circuits are preferred for otherdevices, like hydraulic pistons, where such hydraulic pistons are usedfor fulfilling the main task of forklift trucks, excavators, shoveldozers and so on. Although it is of course possible from a technicalviewpoint that for a machinery that needs to employ an open hydraulicfluid flow circuit, a closed hydraulic fluid flow circuit is arrangedadditionally, such an approach is of course disadvantageous from aneconomic viewpoint. Not only more components have to be foreseen andmaintained for the respective machine, but also the energy efficiencydrops because of the necessarily higher weight of the respectivemachine. Therefore, there is a desire to use a propelling arrangement incombination with an open hydraulic fluid flow circuit, albeit this isnot necessarily the best starting point.

The main problem when using an open hydraulic fluid flow circuit issurprisingly not the propelling task, but instead the main problem iswhen it comes to braking and/or to enable a coasting of the vehicle.Here, one is not only confronted with the problem of how to generate abraking force for the combined hydraulic pump/hydraulic motor that isconnected to the wheels of the vehicle, but one is also confronted withthe problem of how to avoid cavitation on the suction side of thecombined hydraulic pump/hydraulic motor in these modes of operation.

The situation becomes even more problematic if the vehicle has to beable to move in different directions (i.e. forward and backward) and hasto be able to be slowed down in these directions as well (brakingcapability), which is a standard requirement.

Although some suggestions were already made in the state of the art, sofar they were not necessarily satisfying.

One suggestion was to sort of “circumvent the problems” by simplyproviding a braking force with standard mechanical brakes and to avoidcavitation by simply continuing to supply pressurized fluid by thestandard hydraulic pump. However, it is quite obvious that this isdisadvantageous, not only due to the wear of the mechanical brakes, butalso due to the lower energy efficiency of the vehicle.

SUMMARY

Therefore, there is a need for a fluid flow arrangement with which avehicle can be propelled and slowed down although an open hydraulicfluid flow circuit is employed.

It is therefore the object of the invention to suggest a fluid flowarrangement that comprises an adjustable fluid pumping device and afluid working machine that is fluidly connected to said fluid pumpingdevice and that further comprises a re-circulating loop, fluidlyconnecting a first fluid port and a second fluid port of said fluidworking machine, where said first and said second fluid port arepreferably at times at a different pressure level that is improved oversimilar fluid flow arrangements that are known in the state of the art.Another object of the invention is to suggest an electronic controllingdevice for controlling a fluid flow arrangement that is improved overelectronic controlling devices that are known in the state of the art.

The invention solves this object.

It is suggested to design a fluid flow arrangement, comprising apreferably adjustable fluid pumping device, a fluid working machine thatis fluidly connected to said fluid pumping device and a re-circulatingloop that is fluidly connecting a first fluid port and a second fluidport of said fluid working machine, where said first and said secondfluid port of said fluid working machine are preferably at times at adifferent pressure level in a way that the re-circulating loop comprisesa controllable fluid throttling device and a switchable fluid conduitdevice. Although in principle the fluid pumping device and particularlythe fluid working machine can be of a fixed type (so that one revolutionof the respective machine will pump/input essentially the same fluidvolume; minor variations can of course occur due to some effects likepressurization effects, viscosity effects and the like) it is preferredif at least the fluid pumping device or the fluid working machine,preferably both the fluid pumping device and the fluid working machineare of an adjustable type, so that those machines can be varied in a waythat one revolution will pump/input a variable amount of fluid (withincertain limits, of course). How this adjustment is done is arbitrary. Inparticular, fluid pumps and/or fluid working machines can be of a typewere the adjustment is performed by mechanical means. As an example:wobble plate pumps/wobble plate fluid working machines are of such amechanically adjustable type that is well known in the state of the art.However, it is also possible (and even preferred), if such fluid pumpingdevice and/or such fluid working machine is of an electricallyadjustable type. Such an electrically adjustable type is also well knownin the state of the art under the name digital displacement Pump® (DDP)or synthetically commutated hydraulic pump and/or motor (depending onthe exact design). Fluid pumping devices and/or fluid working machinesof the electrically adjustable type have the advantage that they can bevaried much quicker and/or in broader ranges, both is advantageous inparticular if it comes to propelling a vehicle. In a typical applicationof the presently suggested fluid flow arrangement, the fluid pumpingdevice is usually of a design that it can only pump hydraulic fluid,i.e. increasing the pressure of a hydraulic fluid while consumingmechanical energy (typically applied in form of a rotation, i.e. atorque). Of course, it is also possible that the fluid pumping device isof a combined pumping/motoring design (albeit the motoring mode willusually be used only rarely, if at all). The fluid pumping device willusually be connected to a driving device, such as a combustion engine,an electric motor or the like. It is to be noted that if the fluidpumping device (and/or the fluid working machine) is of an adjustabletype, it is possible that an electric motor or a combustion engine canbe running at a certain, constant speed, like the speed of maximumpower, the speed of maximum torque or the speed of maximum energyefficiency (where it is of course possible that a different speed regimeis chosen from time to time, depending on the current operatingrequirements). Due to the adjustability of the fluid pumping device itis nevertheless possible to vary the amount of hydraulic fluid that ispumped by the fluid pumping device. The fluid working machine istypically of a design that it can be switched between a motoring modeand a pumping mode (where a partial motoring and pumping mode can beenvisaged as well, in a way that the fluid working machine comprisesdifferent services that are fluidly separated from each other, andwherein some (at least one) of the services are operated in a motoringmode, while at the same time some (at least one) of the services areoperated in a pumping mode). Additionally, the fluid working machine canbe of a fixed displacement type or of a variable displacement type(where the variable displacement type typically shows some advantageswith respect to controllability of the fluid flow arrangement). In themotoring mode, it is able to actively propel a vehicle, if used in thiscontext. If the vehicle has to be decelerated, the fluid working machinewill be switched to a pumping mode so that pressurized fluid will be“consumed” by other consumers, in particular by the controllable fluidthrottling device which will be elucidated later on. This way wear-freebraking action can be realised. Typically, the fluid working machinewill be connected to one or several devices (either directly or with anymechanical transmission system in-between (including a gear or thelike)). Only for the sake of completeness, it should be mentioned thatit is of course possible to use two or even more (adjustable) fluidpumping devices and/or two or more fluid working machines within thefluid flow arrangement. Depending on the mode the fluid flow arrangementis currently operated in, the first fluid port will be the high pressureport and the second fluid port will be the low pressure port, or viceversa. Typically, in case the fluid working machine is operating in amotoring mode, the high pressure port will be the fluid input port,while the low pressure port will be the fluid output port. If, however,the fluid working machine will be driven in a pumping mode, usually thehigh pressure port will be the fluid output port, while the low pressureport will be the fluid input port. It should be noted that from amechanical viewpoint, the first fluid port can comprise several“mechanical fluid ports”, although the various “mechanical fluid ports”from a single “logic fluid port” (the same applies mutatis mutandis tothe second fluid port). Such a plurality of “mechanical fluid ports” canbe connected together using a manifold or the like. However, it is alsopossible that the fluid working machine shows several internal fluidcircuits that are separated from each other, for example for generatingvarious pressure levels or the like. Again, for the sake of completenessit should be mentioned that the first fluid port and the second fluidport can show the same pressure level at certain times. The most obviouscase for this is if the machine is shut down (for example a forklifttruck that is parked overnight). With the help of the re-circulatingloop the first fluid port and the second fluid port can be selectively“sort of short-circuited” at times. Thanks to this feature, a cavitationat the low pressure side can be effectively avoided. In particular, suchcavitation could otherwise occur especially at the fluid input port ifthe fluid working machine is operated in a pumping mode (which happensif a vehicle is operated in a coasting mode or in a braking mode). Ofcourse, this “sort of short-circuiting” should not always be present,because otherwise quite some losses of pressurized hydraulic fluid wouldoccur if the fluid working machine is operated in a motoring mode whendriving a vehicle. Then, huge energy losses would occur, or the vehiclewould not be driveable anymore, at all. This selective switching of there-circulating loop can be realised by the presently suggestedswitchable fluid conduit device. This switchable fluid conduit devicecan be chosen from a huge variety of devices. In particular, both activeand passive switchable fluid conduit devices are possible. An activeswitchable fluid conduit device could be a solenoid valve, where thesolenoid valve will be actuated by an electric (electronic) controller,for example. However, it is also possible that the switchable fluidconduit device is of a passive type so that no “active actuation signal”has to be generated. A possible design for such a passive switchablefluid conduit device would be a check valve or the like. It is easilyunderstandable that the switchable fluid conduit device should bedesigned, arranged and/or actuated in a way that the re-circulating loopis not closed during intervals (i.e. a fluid flow throughput ispossible, where the fluid working machine is operated in a motoringmode, while the re-circulating loop should be “sort of short-circuited”during intervals when the fluid working machine is operated in a pumpingmode (during coasting and/or braking of a vehicle, for example). By thenotion “sort of short-circuited”, usually a closed fluid loop is meant,where the “sort of” stands for a possible fluid obstruction device,where the obstruction to the fluid flow can be even considerably large.However, it should be low enough that cavitation effects for the fluidworking machine can be avoided. A “sort of short-circuiting” on the “lowfluid flow resistance” side (i.e. essentially fluidly short-circuitingthe re-circulation loop) will essentially result in little (if at all)braking performance. This mode can nevertheless be advantageous forrealising a coasting mode for a vehicle, as an example. In case the“sort of short-circuiting” is on the “high fluid flow resistance” side,a “real braking” mode for a vehicle can be realised. This is because thepressure loss over the fluid flow obstruction device (where themechanical energy stored in the pressure level of the fluid will beconverted to thermal energy) will operate as a non-wearing brake for thefluid working machine. The fluid obstruction device is realised as acontrollable fluid throttling device. The controllability of thecontrollable fluid throttling device can be chosen from a wide range. Inparticular, it is possible that the controllable fluid throttling devicecan be switched between (essentially) two modes only, namely a firstmode, where essentially no fluid flow resistance will be generated(during coasting or during active propelling of a vehicle, for example),and a second mode, where a certain fluid flow resistance will begenerated (for realising a “real braking mode” for a vehicle, forexample). However, it is preferred if a plurality of different states,in particular a continuous range of “fluid obstruction strengths” can berealised by the controllable fluid throttling device. This way, variousbraking strengths, preferably a continuous range of braking performancecan be realised. Once again for the sake of completeness: a mode, wherethe controllable fluid throttling device exerts essentially no fluidflow resistance on the hydraulic fluid flowing through it, is importantbecause otherwise such a fluid flow resistance would be present during amotoring mode of the fluid working machine, as well. Otherwise, hugeenergy losses would occur if a vehicle is propelled, as an example. Inparticular, the controllability of the controllable fluid throttlingdevice can be realised by a throttling device that has an orifice of avariable size. However, different designs are possible as well. As anexample, a device could be used where a tube of a certain diameter isemployed and where the “effective” length of the tube (as seen by thecirculating fluid) can be changed by “adding or removing” additionalloops using switchable valves. However, other designs are possible aswell.

It is suggested to design the fluid flow arrangement in a way that saidre-circulating loop can be circulated in opposing directions, inparticular in a way that it comprises at least two controllable fluidthrottling devices and/or at least two switchable fluid conduit devices.This way, it is possible to operate the fluid working machine inopposite directions (where the fluid pumping device is normally usedonly in one direction; however, it might occur as well that even thefluid pumping device can be operated in alternating directions, at leastat times). Using this design, a vehicle that is propelled by thepresently suggested fluid flow arrangement (in particular if the wheelsare mechanically connected to the fluid working machine) can be drivenin different directions, i.e. in a forward direction, as well as in arearward direction. Such a design thus yields an increased operabilityof the respective machinery. A fluid flow circulation in opposingdirections in the re-circulating loop can be very effectively realisedwith the presently suggested design of at least two controllable fluidthrottling devices and/or at least two switchable fluid conduit devices.In particular, when such a design is used, the functionality of acoasting and/or braking mode can be easily realised in both drivingdirections of the respective vehicle.

Another preferred embodiment of the fluid flow arrangement can beachieved if said adjustable fluid pumping device and said fluid workingmachine are connected using at least one fluid switching means, inparticular in a way that the output of said fluid pumping device can beselectively connected to at least one of said at least two differentfluid ports of said fluid working machine, in particular to one of saidfirst and said second fluid port of said fluid working machine. Usingthis embodiment, it is particularly simple to realise an operation ofthe fluid working machine in two different directions. If used inconnection with propelling a vehicle, a forward and a rearward movementof the vehicle can be easily achieved. Furthermore, using the proposeddesign it is particularly simple to split the fluid flow generated bythe fluid pumping device and the return flow from the fluid workingmachine from each other, if the fluid flow arrangement is operated in apropelling mode.

It is further suggested to design the fluid flow arrangement in a waythat said fluid flow arrangement is an open hydraulic fluid flowcircuit. In particular, it is suggested to use the fluid flowarrangement for propelling purposes, especially for propelling landvehicles. Using the proposed design, the presently proposed fluid flowarrangement can show its intrinsic advantages and features particularlywell.

Another preferred design of the fluid flow arrangement can be achievedif at least one of said controllable fluid throttling devices isdesigned as a pressure relief valve with a preferably adjustable setpoint. As already stated above, the adjustability of the set point canbe in a way that two, three, four or even more, i.e. a plurality ofdifferent discrete states can be realised. However, it is also possible(and usually preferred), if the set point can be adjusted continuously(within a certain interval). Usually, the adjustability of the set point(control of the fluid throttling device in general) is done in anautomated way. How the input signal is applied is generally withoutrelevance. As an example, the adjustment signal could be applied bymechanical means, by electrical means and/or by fluid means (pneumatics,hydraulics). Usually, an electric control signal is preferred, sincesuch a signal can be easily generated by an electric control device (inparticular an electronic controlling device). It should be stated thatit is of course possible as well to apply two or even more controlsignals, where each individual control signal has a certain influence onthe setting of the controllable fluid throttling device/the adjustableset point. By a concurrence of the individual control signals, the“final setting”/“final set point” will be realised.

Yet another preferred design can be realised if said controllable fluidthrottling device is an electrically controllable device and/or if saidcontrollable fluid throttling device is controlled by an electroniccontrolling device, in particular by a programmable electroniccontrolling device. As already mentioned above, the generated controlsignal can be a singular one. However, a plurality of control signalscan act on the fluid throttling device as well. If the fluid throttlingdevice is an electrically controllable fluid device, it is usually veryeasy to realise a fast and precise actuation of the controllable fluidthrottling device, resulting in the typically good controlling behaviourof the fluid flow arrangement. Furthermore, the generation of anelectric control signal is typically quite easy to achieve. As proposed,the control can be performed (in part) by an electronic controllingdevice, in particular by a programmable electronic controlling device. Apreferred design for this is an electronic microcontroller. Inparticular, a controlling device that is designed as a single printedcircuit board device is preferred. Such devices are easily and cheaplyavailable in the state of the art. Just to name an example, a RaspberryPi © or an Arduino controller © are available for little money and showa remarkable computing power in the meantime. The electric control of anelectrically controllable fluid throttling device can be realised as anelectric coil, generating a magnetic force on some kind of a spool orthe like. Also, it is possible to use a stepper motor or an electricmotor (including rotational motors as well as linear motors) to create amovement of an appropriate device. In particular, the size of an orificecould be changed within the controllable fluid throttling device.

Yet another preferred fluid flow arrangement can be realised if at leastone of said controllable fluid conduit devices is a directional valve,in particular a check valve and/or if at least one of said controllablefluid conduit devices shows a defined pressure loss behaviour over saidfluid conduit device that is dependent on the fluid flow rate throughsaid fluid conduit device. Using such a design, a reliable and cheapfluid flow arrangement can be realised. If the connection betweenpressure loss and fluid flow rate through the device is known, it ispossible to measure (or at least to estimate sufficiently well) thefluid flow rate by performing pressure measurements. Pressure sensorsfor this are comparatively cheap, need little building space and arequite reliable, even with respect to deteriorated hydraulic oil due toalteration effects. Furthermore, since the controllable fluid conduitdevice is needed anyhow and it is essentially impossible to avoid apressure loss over it, this pressure loss can be used for a sensiblepurpose. In particular no additional fluid flow resistance has to beintroduced for the fluid flow arrangement which result in a higherenergy efficiency and generally a better performance of the fluid flowarrangement.

A further preferred embodiment can be realised, if the fluid flowarrangement comprises at least one pressure measuring device, inparticular a plurality of pressure measuring devices that are preferablyarranged at the re-circulating loop, more preferably between a fluidport of said fluid working machine and at least one of said controllablefluid throttling devices and/or between at least two of saidcontrollable fluid throttling devices and/or at the fluid output line ofa preferably adjustable fluid pumping device. Using such pressuremeasuring devices, it is possible to gain sufficient knowledge forcontrolling the behaviour of the fluid flow arrangement in asufficiently precise way. In particular, using pressure measuringdevices one can obtain more information about the amount of fluid thatis pumped by the fluid working machine in a coasting operation or abraking operation. Using this additional input, the operation of thefluid flow arrangement, in particular of the braking behaviour of thefluid flow arrangement, can be controlled more precisely. Thus, it iseven easily possible to mimic the behaviour of a standard mechanicalbrake or the braking behaviour of a dedicated closed hydraulic fluidflow circuit. When talking about placing (one of) the pressure measuringdevice(s) at the fluid output line of a preferably adjustable fluidpumping device, this should be mainly understood in a logical sense.Therefore, placing the pressure measuring device near and/or in thevicinity of the fluid working machine is of course possible (and quiteoften even advantageous, since due to the position near the fluidworking machine the measured pressure will usually better reflect thepressure level near the fluid working machine). However, sometimes itmight be advantageous as well to place the pressure measuring device (orpossibly even an additional one) close to the fluid output port of thepreferably adjustable fluid pumping device.

According to another aspect of the invention, it is proposed to designan electronic controlling device for controlling a fluid flowarrangement, in particular for controlling a fluid flow arrangement ofthe previously described design, wherein the fluid flow arrangementcomprises at least one fluid working machine, at least a re-circulatingloop, fluidly connecting a first fluid port and a second fluid port of afluid working machine and at least one controllable fluid throttlingdevice that is arranged in said re-circulating loop in a way that saidelectronic controlling device generates a control signal for said atleast one controllable fluid throttling device in a way to generate adefined decelerating force for said fluid working machine. Theelectronic controlling device can be particularly a microprocessorand/or a single printed circuit board controller. As already mentioned,an Arduino © controller or a Raspberry Pi © could be used for thispurpose. Using such an electronic controlling device it is possible tomimic the behaviour of a standard mechanical brake for a vehicle with ahydraulic circuit, in particular even with an open hydraulic fluid flowcircuit. In particular, a variety of different operating modes can beeasily implemented using the electronic controlling device. Theelectronic controlling device can be a dedicated electronic controllingdevice that is more or less solely used for operating the fluid flowarrangement. However, the electronic controlling device can be likewisea device that implements more functionality of the machinery, the fluidflow arrangement is used for. In this case, a sufficient amount ofcomputing power has to be provided for fulfilling the computations,necessary for operating the fluid flow arrangement.

In particular, it is possible that the electronic controlling device isdesigned in a way that at least one sensor signal, describing thecurrent state of the fluid flow arrangement, is used for generating saidcontrol signal. In particular, it is possible that pressure data is usedfor generating said control signal. Of course, other sensor signals canbe used additionally and/or alternatively for generating said controlsignal. Pressure data can be particularly obtained from pressuremeasuring devices. The use of such data (with which even a fluid flowcan be determined “indirectly”) was previously suggested. Furthermore,not only sensor signals can be used that come from sensors that are moreor less only provided for the purpose of operating the fluid flowarrangement, but instead sensor signals that come from sensors that areprovided for a different purpose (for example for operating a combustionengine that drives the fluid pumping device) can be used as well. Evenmore, other data that is present anyhow (for example some values thatcome from the present electronic controlling device or from anotherelectronic controlling device for any purpose whatsoever) can be used asan input signal as well.

It is further suggested to design the electronic controlling device in away that said control signal is generated in a way that the fluid flowarrangement can be operated in at least one mode, taken from the groupcomprising: a method, in which the speed of the fluid working machine iscontrolled by outputting an appropriate control signal to control thepressure at an outlet port of the fluid working machine, while the fluidworking machine is not driven by a fluid pumping device; a method, inwhich the speed of the fluid working machine is controlled by outputtingan appropriate control signal to control the pressure at the outlet portof the fluid working machine, while the fluid working machine is driven,at least in part, by a fluid pumping device; and a method where theturning direction of the fluid working machine is reversed by firstslowing down the speed of the fluid working machine and then switching afluid switching means in a way that the output of a fluid pumping deviceis selectively connected to a different fluid port of said fluid workingmachine. Using such an embodiment (or a combination thereof) aparticularly versatile device can be realised. In particular, a coastingand braking operation can be realised that shows various possiblyadvantageous embodiments. As an example, if the fluid working machine isnot driven by a fluid pumping device and the speed of the fluid workingmachine is controlled by outputting an appropriate control signal tocontrol the pressure at the outlet port (i.e. where usually the controlof the speed of the fluid working machine is done by setting anappropriate pressure level at the fluid outlet port of the fluid workingmachine, which in turn is usually done by setting an appropriatepressure difference over a fluid throttling device that is arranged aftof the fluid outlet port of the fluid working machine), a non-wearingbraking system can be realised that is highly energy-efficient (nomechanical work is needed during the operation of the arrangement). If,however, the fluid working machine is controlled by outputting anappropriate control signal to control the pressure at the outlet portwhile the fluid working machine is driven, it is possible to determinethe speed of the fluid working machine by the speed of the fluid pumpingdevice, and hence by the combustion engine (to give an example). Thismight be advantageous if only a short braking impulse is needed to avoida rapid deceleration and acceleration of the turning speed of thecombustion engine, which might be a nuisance to the operator of thedevice. Furthermore, it might be desired by the operator to have an“audible feedback” of the driving speed by means of the turning speed ofthe combustion engine. When employing a method, where the turningdirection of the fluid working machine is reversed by first slowing downthe speed of the fluid working machine and then switching a fluidswitching means in a way that the output of a fluid pumping device isselectively connected to a different fluid port of the fluid workingmachine, a behaviour of the vehicle (as an example) is achievable whichis very comfortable (and even safe, for example in the case of aforklift truck, where goods could fall off the fork in the case of arapid deceleration/acceleration of the forklift truck). In particular, avery strong braking behaviour can be avoided when the reverse gear isselected, while the vehicle is still moving forward. If the fluidswitching means would be operated so that the output of the fluidpumping device is connected to the respective different fluid port whilethe vehicle is still in motion, this would essentially unavoidably leadto the effect that a very strong braking force is applied until thevehicle comes to a complete stop. It is easily understandable that sucha behaviour is not necessarily what is desired.

A further proposal is to design a fluid flow arrangement, in particulara fluid flow arrangement according to the previous description in a waythat it shows an electronic controlling device according to the previousdescription.

Yet another advantageous embodiment can be obtained, if the fluid flowarrangement is used as a propelling means for a vehicle, in particularfor a land vehicle. In this case, the fluid flow arrangement can showits intrinsic advantages and features particularly well.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features, and objects of the invention will beapparent from the following detailed description of the invention inconjunction with the associated drawings, wherein the drawings show:

FIG. 1: the schematic circuitry of a first embodiment of a hydraulicpropelling circuit, employing an open hydraulic fluid flow circuit;

FIG. 2: the first embodiment of a hydraulic propelling circuit in adriving mode;

FIG. 3: the first embodiment of a hydraulic propelling circuit in arunaway prevention mode, in which the turning speed of the drivingengine is independent from the turning speed of the fluid workingmachine;

FIG. 4: a possible control schematic for implementing a runawayprevention mode;

FIG. 5: the first embodiment of a hydraulic propelling circuit in ametering mode, in which a braking is effectuated wherein the turningspeed of the fluid working machine depends on the turning speed of thedriving engine;

FIG. 6: a possible control schematic for implementing a metering mode;

FIG. 7: the first embodiment of a hydraulic propelling circuit in aningressive reversal mode that is to be avoided;

FIG. 8: the schematic circuitry a second embodiment of a hydraulicpropelling circuit.

DETAILED DESCRIPTION

In FIG. 1, a first embodiment of a hydraulic propelling circuit 1 thatcan be used for moving a vehicle (in particular a vehicle that uses ahydraulic system anyhow, for example a forklift truck, a shovel loader,an excavator or the like) is shown as a fluid flow schematic. Thepresently shown hydraulic propelling circuit 1 is set up in a way thatthe vehicle can be moved in two different (opposing) directions, i.e. ina forward and in a backward direction. Since the schematic is set up ina symmetric way, the propelling characteristics (maximum speed, torqueetc.) are essentially identical in both directions. This is actually apreferred behaviour for machinery such as excavators or forklift trucks.Furthermore, it can be seen that the hydraulic propelling circuit 1 isof an open hydraulic fluid flow circuit type.

In “reality”, the output of the main hydraulic pump 2 could be used fordifferent purposes as well, like for hydraulic pistons for raising thefork of a forklift truck, for moving the shovel of a shovel loader orthe like. Of course, it is also possible that a dedicated pump for such“other hydraulic services” is used (or presumably a main hydraulic pump2 is used that comprises different independent services, where differentservices are used for different hydraulic sub-circuits).

The main hydraulic pump 2 is driven by a driving engine, which ispresently designed as a combustion engine 3 (for example a diesel motoror a natural gas motor). The torque that is generated by the combustionengine 3 is transmitted by a driving shaft 4 to the main hydraulic pump2.

As can be seen as well, an auxiliary hydraulic pump 5 is provided. Theauxiliary hydraulic pump 5 pumps hydraulic fluid from an oil reservoir 6(typically at ambient pressure) to the low pressure side 8 of thehydraulic propelling circuit 1. Furthermore, the auxiliary hydraulicpump 5 can additionally serve as a source of fluid for othertasks/devices (presently not shown). In particular, throttling valvescan be used to provide different pressure levels for such additionaltasks/devices and/or for the elevated pressure in the low pressure side8 of the hydraulic propelling circuit 1, in particular in case a singleauxiliary hydraulic pump 5 is used.

Both the main hydraulic pump 2 and the auxiliary hydraulic pump 5 (orpossibly other additional hydraulic pumps that are not shown, as well)take in hydraulic oil from the fluid reservoir 6.

A minimum pressure is guaranteed by means of the auxiliary hydraulicpump 5, so that the respective fluid lines 26 do not run dry. On theother hand, the pressure within the low pressure side 8 of the hydraulicpropelling circuit 1 is limited to a comparatively low pressure by meansof a low pressure relief valve 9, that can be designed as a (slightly)pre-loaded check valve (where the preloading can be realised by ahelical spring or the like) as it is well known in the state of the art.

With “realistic set-ups”, the same combustion engine 3 is used for boththe main hydraulic pump 2 and auxiliary hydraulic pump 5 (typically,both hydraulic pumps 2, 5 are connected to the main driving shaft 4).While it is possible that two separate hydraulic pumps 2, 5 are used,the hydraulic pumps 2, 5 can also be of a “type separated by employingdifferent services”, i.e. they can be designed as several independentservices of common pump housing.

In the presently shown example, the auxiliary hydraulic pump 5 is of afixed displacement type (where the pumping rate of the auxiliaryhydraulic pump 5 is rather limited; the pressure the auxiliary hydraulicpump 5 has to be able to reach is rather limited as well, since only apressure level that is typical for the low pressure side 8 of thehydraulic propelling circuit 1 has to be reached or (slightly)exceeded).

The main hydraulic pump 2 is of an adjustable type, for example avariable displacement hydraulic pump type (a wobble plate pump, forexample). Another (typically preferred) design of the adjustable mainhydraulic pump 2, and that is chosen for the presently shown embodiment,is a so-called digital displacement pump (DDP®), that is also known as asynthetically commutated hydraulic pump in the state of the art.

The pressurized fluid that is pressurized by the main hydraulic pump 2is fed to the high pressure side 7 of the hydraulic propelling circuit1. Using an appropriate switching of the switchable fluid valves 10, 11(both of an on-off-type), the pressurized fluid can be fed to eitherport “A” (via fluid valve 10) or to fluid port “B” (via fluid valve 11)of the fluid working machine 12. The fluid working machine 12 is acombined fluid motor/fluid pump. It can be of a purely mechanicalnature, or it can be controlled by appropriate controlling signalsand/or can send sensor signals to an electronic controller 13 viaelectric signal lines 14. The electronic controller 13 is not onlyconnected to the fluid working machine 12 by means of electric signallines 14, but also other components of the hydraulic propelling circuit1 are connected to the electronic controller 13 by means of electricsignal lines 14 for obtaining control signals and/or for feeding sensorsignals (or other feedback signals) to the electronic controller 13. Inparticular, the already described combustion engine 3, main hydraulicpump 2, fluid valves 10, 11 and fluid working machine 12 are connectedto the electronic controller 13.

As can be seen from the schematic as well, pressure sensors 16, 17 arefluidly connected to appropriate fluid lines 26 for monitoring thepressure in the respective part of the hydraulic propelling circuit 1.The pressure values that are measured by the respective pressure sensors16, 17 are fed to the electronic controller 13. Namely, pressure sensor16 is located aft of fluid valve 10 in the vicinity of Port “A” of thefluid working machine 12, while pressure sensor 17 is located aft offluid valve 11 in the vicinity of Port “B” of the fluid working machine12.

The middle part 18 of the hydraulic propelling circuit 1 (where thefluid working machine 12 is located) is connected to the low pressureside 8 by means of a valve combination 19, 20 that is arranged either onthe “A” side (right side) or the “B” side (left side) of the hydraulicpropelling circuit 1. Namely, the valve combination 19 on the right sidecomprises an adjustable pressure relief valve 21 that allows a fluidflow from middle part 18 to low pressure side 8 of the hydraulicpropelling circuit 1, in case an appropriate pressure difference ispresent. The cracking pressure of adjustable pressure relief valve 21can be adjusted by electronic controller 13 by applying an appropriateactuation signal via the appropriate electrical signal line 14. Thus,the pressure difference between pressure sensor 16 (hydraulic pressurein the middle part 18 in the proximity of Port “A” of the fluid workingmachine 12) and pressure sensor 25 (hydraulic pressure in the lowpressure side 8 of hydraulic propelling circuit 1) can be set to adefined value (of course, typically only if the pressure at pressuresensor 16 is higher than the pressure at pressure sensor 25).

If the pressure difference reverses (i.e. pressure at pressure sensor 25is higher than pressure at pressure sensor 16) a check valve 23 opensand a fluid flow is permitted from the low pressure side 8 to the middlepart 18 of the hydraulic propelling circuit 1.

The valve combination 20 of the arrangement on the “left side” of thehydraulic propelling circuit 1 (near port “B” of the fluid workingmachine 12) is done in a similar way as on the “right side”. Inparticular, the valve combination 20 comprises an adjustable pressurerelief valve 22 and a check valve 24 whose operation and functionalityis similar to the valve combination 19 on the “right side” and detaileddescription is omitted for brevity.

Of course, the pressure level on the low pressure side 8 of thehydraulic propelling circuit 1 that is measured by pressure sensor 25 isfed to the electronic controller 13 by an appropriate electric signalline 14 as well.

In FIG. 2, a “standard driving situation” of the hydraulic propellingcircuit 1 is shown. In particular, the direction of the fluid flow isindicated by arrows 27 near the respective hydraulic fluid line 26. Inthe presently shown example, the fluid working machine 12 is rotating inone direction (for example a forward direction of a forklift truck, ifthe hydraulic propelling circuit 1 is used for such a forklift truck).If the moving direction of the fluid working machine 12 (and thence ofthe forklift truck) has to be reversed, the fluid flow will be changedby establishing a fluid flow in a way that essentially the left side(“B”) and the right side (“A”) of the hydraulic propelling circuit 1near fluid working machine 12 are interchanged.

In the “standard driving mode”, hydraulic fluid is sucked in from thefluid reservoir 6 by the main hydraulic pump 2, pressurized and ejectedtoward the high pressure side 7 of the hydraulic propelling circuit 1.The fluid valves 10, 11 are switched in a way that a fluid connection isestablished between high pressure side 7 and Port “A” of fluid workingmachine 12 in the middle part 18 of hydraulic propelling circuit 1. Thefluid connection between the high pressure side 7 and the side of themiddle part 18 near fluid port “B” of the fluid working machine 12,however, is disconnected. Therefore, fluid valve 10 is switched “on”(permitting a fluid flow there through), while fluid valve 11 is “off”(no fluid flow permitted through the valve).

Since no braking performance is needed, the adjustable pressure reliefvalve 22 of the valve combination 20 on the “left side” (“B”-side) isset to a mode that the pressure difference across the valve is 0 (apartfrom unavoidable residual effects). In effect, setting the pressuredifference to essentially 0 is advantageous from an energetic viewpoint,since any pressure difference over adjustable pressure relief valve 22would result in a fluid obstruction resulting in reduced energyefficiency of the system.

Of course, to avoid some kind of “short-circuiting”, the adjustablepressure relief valve 21 of the valve combination 19 on the “right side”(“A”-side) is set to its maximum value, so that as a consequence anyfluid flow through adjustable pressure relief valve 21 is hindered(apart from the possibility of any “emergency depressurization” due to adefect of the arrangement).

As can be seen by the appropriate arrows 27 near the hydraulic fluidlines 26, the hydraulic fluid is therefore directed via fluid valve 10(right side), through the fluid working machine 12 (direction port“A”→“B”), adjustable pressure relief valve 22 (left side), low pressurerelief valve 9 back to the fluid reservoir 6.

Therefore, mechanical energy that comes from the combustion engine 3 isconverted to pressurization energy by the main hydraulic pump 2, whichis converted back to mechanical energy at the fluid working machine 12(operating as a hydraulic motor in this mode of operation).

This will result in a positive torque which accelerates the fluidworking machine 12 and the attached load (for example for propelling avehicle).

Apart from small amounts of a leakage in the hydraulic propellingcircuit 1 and its components, the hydraulic fluid flow across the fluidworking machine 12 can be assumed to be identical to the hydraulic fluidflow through the main hydraulic pump 2. Hence, with a known displacementof the fluid working machine 12, the speed of the fluid working machine12 (and hence of the load, for example the speed of a vehicle) can becontrolled by controlling the fluid output flow of the main hydraulicpump 2.

Now, if the amount of fluid that is pumped by the main hydraulic pump 2is reduced to (approximately) 0, the fluid flow behaviour according toFIG. 3 will be established.

Due to the switching-off of main hydraulic pump 2, no fluid flow isdelivered to the fluid working machine 12 (by the main hydraulic pump 2)anymore. In effect, fluid valve 10 could be switched off as well.

Now the problem would arise that cavitation occurs in part of the“A”-side of the hydraulic propelling circuit 1. Such a cavitation has tobe avoided, since it could seriously damage the respective components,in particular the fluid working machine 12. Therefore, the hydraulicpropelling circuit 1 is designed in a way that a fluid back flow to port“A” of the fluid working machine 12 is possible. Please note that rightat the moment the (actuated) fluid valves 10, 11 and/or the adjustablepressure relief valves 21 and 22 are still at the setting according tothe situation shown in FIG. 2.

In consequence, a “short-circuited” fluid flow is established, startingfrom port “B” (the fluid output port of fluid working machine 12, whichnow works as a hydraulic fluid pump) through “left” adjustable pressurerelief valve 22 (pressure difference set to 0), through “right” checkvalve 23 (pressure difference across the check valve 23 is 0 as well)and back to port “A” (fluid intake port) of the fluid working machine12.

Now, obviously, the connection between the turning speed of thecombustion engine 3 and/or the main hydraulic pump 2 and the turningspeed of the fluid working machine 12 is lost. In particular, thecombustion engine 3 and/or the main hydraulic pump 2 could be idling,while the fluid working machine 12 is still running at an elevated speed(in case the hydraulic propelling circuit 1 is used for propelling avehicle, the vehicle would still move).

This situation can be voluntarily (desired mode of operation), as in thecase of idling the main hydraulic pump 2 while coasting the fluidworking machine 12 (coasting a vehicle). However, the situation couldalso be involuntarily, as in the case of “running away” downhill of avehicle.

Now, some kind of a braking capability has to be implemented. This isdone by setting the “left” adjustable pressure relief valve 22 to acertain pressure differential that corresponds to a certain, desiredbraking behaviour (“runaway prevention mode”). Typically, the “right”adjustable pressure relief valve 21 will remain at a setting (will beset to a setting) of a maximum pressure difference (effectively, aswitched off-condition).

From a controlling side, the situation according to FIG. 3 (“runawayprevention mode”) can be identified by the electronic controller 13 bythe first condition that P_(B)>P_(A) (pressure P_(B)=pressure at “left”pressure sensor 17; “B”-side, while pressure P_(A)=pressure at the“right” pressure sensor 16 at the “A”-side). This can be easilyunderstood because the pressure at the “A”-port of the fluid workingmachine 12 drops to 0 (hopefully not below 0 because of possiblecavitation), while due to the pumping behaviour of the fluid workingmachine 12, the “B”-port will be at a certain pressure level (becausethere will always be some pressure due to fluid obstructions and fluidviscosity).

Another condition for detecting the situation according to FIG. 3 is theabsence of a fluid flow (fluid flux) through the main hydraulic pump 2(Q_(MHP)=0). This can be seen by the actuating signal to the mainhydraulic pump 2.

To establish a defined braking behaviour for the hydraulic propellingcircuit 1 (“runaway prevention mode”), “left” adjustable pressure reliefvalve 22 has to be set to a certain point, so that the pressure at fluidport “B” of the fluid working machine 12 reaches a certain point. Then,the fluid working machine 12 has to work against a pressure differenceP_(B)−P_(A), so that the fluid working machine 12 has to perform somemechanical work against the difference in pressure level; which isequivalent to a braking power performed on the fluid working machine 12(and possibly the vehicle's movement, if employed for this use).

A possible control schematic for this is shown in FIG. 4.

The input value P_(C)−ΔP_(maximum) allowable 28 is the allowablepressure difference over “right” check valve 23. Since the check valve23 (likewise the “other check valve” 24) is chosen in a way that theconnection between the fluid flow through the respective valve and thepressure difference occurring between both sides of the valve is known,it is possible to determine from this pressure difference over the valvethe fluid flow through the valve (at least in good approximation). This,however, is an indication of the vehicle's speed (if the hydraulicpropelling circuit 1 is used for propelling a vehicle).

This value is fed (at the negative input line) to a comparator 29, whereit is compared with the measured pressure P_(C) at pressure sensor 25that is connected to the low pressure side 8 of the hydraulic propellingsystem 1 (and which is fed into the positive input line of comparator29). The output of the comparator 29 is a value P_(A, set point) 30,namely the “theoretical value” of pressure P_(A), how it should be. Thisis compared to the real value of P_(A) 31 (measured value), i.e. thevalue that is actually measured by “right” pressure sensor 16. This isdone by feeding the respective values into another comparator 32, whoseoutput signal is one of the input signals for the electronic controller13. The electronic controller 13 finally calculates the value P_(PRV)33, which is the pressure set point for the pressure relief valve,currently the “left” pressure relief valve 22. This again is the “maininput value” determining the braking performance of the arrangement.

This way, a wear-free brake can be realised in a simple and efficientway.

Only for completeness it should be noted that a mechanical brake shouldstill be provided for safety reasons, of course.

Another mode that can be realised with the present arrangement (beingdifferent from the previously described “runaway prevention mode”) isthe so-called “metering mode” that is indicated in FIG. 5. Again, thesetting of the fluid valves 10, 11 and pressure relief valves 21, 22 isinitially done in the same way as it is done in FIG. 2.

Now, however, the idea is to realise a braking performance of thehydraulic propelling circuit 1, while maintaining a direct connectionbetween the turning speed of the main hydraulic pump 2 (and hence of thecombustion engine 3 due to the mechanical connection by driving shaft4). Hence, the control of the vehicle's speed is done through anappropriate setting of the adjustable main hydraulic pump 2.

The condition when “metering” can be used (and how it can be identified)is in one respect identical to the previously described “runawayprevention mode”, namely in that P_(B)>P_(A) (fluid working machine 12is operating as a fluid pump, thus performing mechanical work againstthe pressure difference and thus slowing down the vehicle). Different tothe “runaway prevention mode” as previously described, the fluid flowrate of the main hydraulic pump 2 is different from 0 (QMHP≠0).

In order to establish the direct correspondence between fluid flowthrough the fluid working machine 12 (and hence rotational speed of thefluid working machine 12) and the fluid flow, generated by the mainhydraulic pump 2, the pressure upstream of the fluid working machine 12(which is the pressure at pressure port “A”, i.e. P_(A)) must bemaintained at a sufficiently high level to not only avoid cavitation,but also to avoid re-circulating fluid flow, presently through “right”check valve 23. This translates to the requirement for pressure P_(A)upstream of the fluid working machine 12 to be higher than the pressureP_(C) in the low pressure side 8 (measured by pressure sensor 25), i.e.higher than P_(C). (“Right” adjustable pressure relief valve 21 is keptat a “closed” condition, i.e. at maximum pressure difference setting).

An appropriate control scheme schematic for this is shown in FIG. 6. Nowone of the input values of first comparator 29 is changed to A P nocirculation 34, i.e. to a setting so that pressure P_(A) near inlet port“A” of the fluid working machine 12 is maintained at a level that ishigher than P_(C) in the low pressure side 8 of the hydraulic propellingcircuit 1. This is compared to P_(C) 35, as measured by pressure sensor25. Contrary to the previous case, however, comparator 29 uses bothvalues 34, 35 as a positive input signal. The output 36 of firstcomparator 29 is now P_(C)+ΔP_(no circulation) as a set point. This iscompared as in the previous case with measured value of P_(A) 31, asmeasured by presently “right” pressure sensor 16 by comparator 32. Thisis the input signal for electronic controller 13, which calculates as anoutput signal the pressure set point P_(PRV) 33 for the presently “left”pressure relief valve 22 (therefore, the set point for this pressurerelief valve will change from the initial “0-setting”).

Although in the examples of FIGS. 2, 3 and FIG. 5 a (let's say) forwardmotion of the vehicle was shown, it is obvious how to realise a backwardmotion by sort of interchanging the fluid flow between the left side andthe right side of fluid working machine 12 and the respective hydraulicfluid lines 26 serving fluid ports A and B.

Nevertheless, a potential problem that still has to be discussed is aproblem that occurs if the hydraulic propelling circuit 1 is switched toa backward moving mode, while the vehicle is still moving forward (orvice versa). This is the problem of “aggressive reversal” which is shownin FIG. 7.

If switching “normally” from a forward to a backward mode of the fluidworking machine 12, this would mean that “right” fluid valve 10 would beswitched from “on” to “off”, while “left” fluid valve 11 would beswitched from “off” to “on”. Furthermore, the initial settings for theadjustment pressure relief valves 21, 22 would be realised, namely asetting that “right” adjustable pressure relief valve 21 would be set to0-pressure difference (from “max”), while “left” pressure relief valve22 would be set to maximum pressure difference setting (from a0-pressure difference; essentially to a shut-off of the respective valvewith the exception of the “emergency function” if the maximum allowablepressure is exceeded). As it is easily understandable, in particular thesetting of “left” adjustable pressure relief valve 22 will lead to amaximum braking power of the hydraulic propelling circuit 1. This wouldresult in an at least uncomfortable behaviour of the vehicle; quiteoften even in a dangerous behaviour, since in the case of a forklifttruck, heavy goods might fall off the fork, resulting in a damage ordestruction of the goods and possibly even in injuries or fatalities ofa person standing nearby. This, of course, is to be avoided.

The idea for solving this problem is to program the electroniccontroller 13 in a way that in case a reversal of direction iscommanded, the electronic controller 13 will at first switch to either“runaway prevention mode” according to FIG. 3, or to “metering mode”according to FIG. 5 and perform a braking action. As soon as a completestop is detected (which can be determined by an equality of pressuresP_(A) and P_(B) at the fluid ports “A” and “B” of fluid working machine12), the “runaway prevention mode” or the “metering mode” will bestopped and the “standard driving mode” as shown and described withrespect to FIG. 2 will be established (in the opposite direction). Thisway, a smooth transition can be made. In particular, it is possible touse a moderate braking power for the “slowing down phase” before areversal of movement is established.

Of course, it should be mentioned that (some of) the pressure sensors16, 17, 25 can be arranged at a different position and/or that someadditional pressure sensors can be provided in the hydraulic propellingcircuit 1 as well. In such a case, the control schematic has to beadapted appropriately (in particular some variations from the embodimentof a control schematic as shown in FIG. 4 and/or in FIG. 6 have to beemployed).

Finally, with respect to FIG. 8, a second embodiment of a hydraulicpropelling circuit 15 is shown as fluid flow schematic. Contrary to thepreviously described embodiment, which enables reversal of motion, thepresently shown embodiment of a hydraulic propelling circuit 15 can onlybe used in one direction (a backward movement has to be realised by someother devices, if needed). As an example, a mechanical gearbox could beintroduced between fluid working machine 12 and the wheels, or a smallelectric helper motor could be used for realising a backward movement.The presently shown embodiment might prove to be useful if no backwardmovement is needed at all, or if a backward movement is used onlyrarely, so that some additional components with very small dimensionscan be used for such a backward movement. This could be the case for anormal car, where a backward movement is used only rarely.

As can be seen from the circuitry scheme, no switchable fluid valves areneeded anymore between the high pressure side 7 and the middle part 18.On the contrary, a simple hydraulic fluid line 26 between main hydraulicpump 2 and fluid working machine 12 is sufficient. Nevertheless, allthree pressure sensors 16, 17, 25 are still used.

Between middle part 18 and low pressure side 8 of the hydraulicpropelling circuit 15, only one pressure relief valve 22, namely theformer “left” pressure relief valve 22 is used, while on the “rightside” only a check valve 23, namely the former “right” check valve 23 isused. The other former “right” pressure relief valve 21 and the former“left” check valve 24 can be omitted, however.

As can be seen, all of the normal driving mode, runaway prevention modeand metering mode can be realised with a simplified circuitry accordingto the second embodiment of a hydraulic propelling circuit 15, if onlyone direction of movement has to be realised. It is understandable, thatdue to the reduced amount of components needed, this hydraulicpropelling circuit 15 is cheaper to implement.

Due to the close similarity of both embodiments of a hydraulicpropelling circuit 1, 15, similar reference numbers have been used forsimilar parts. This does not mean that in real embodiments, therespective components had to be exactly the same.

In particular in the presently described embodiment according to FIG. 8,a sufficient supply of hydraulic oil at the fluid input port A of fluidworking machine 12 during coasting (or breaking) in a way so that nocavitation occurs, can be realised as well by moving the “right” checkvalve 23 in parallel to the main hydraulic pump 2 (with an appropriateopening direction of the check valve). Of course, an additional checkvalve “on top of right check valve 23” can be used as well at theposition of the main hydraulic pump 2.

The same idea can be applied mutatis mutandis to the first embodiment ofa hydraulic propelling circuit 1 as shown and described with referenceto FIGS. 1 to 7 (and likewise to other embodiments as well).

While the present disclosure has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisdisclosure may be made without departing from the spirit and scope ofthe present disclosure.

What is claimed is:
 1. A fluid flow arrangement, comprising a preferablyadjustable fluid pumping device, a fluid working machine fluidlyconnected to said fluid pumping device and a re-circulating loop,fluidly connecting a first fluid port (A) and a second fluid port (B) ofsaid fluid working machine, where said first (A) and said second fluidport (B) are preferably at times at a different pressure level, whereinthe re-circulating loop comprises a controllable fluid throttlingdevice, and a switchable fluid conduit device.
 2. The fluid flowarrangement according to claim 1, wherein said re-circulating loop canbe circulated in opposing directions, in particular in that it comprisesat least two controllable fluid throttling devices and/or at least twoswitchable fluid conduit devices.
 3. The fluid flow arrangementaccording to claim 1, wherein said adjustable fluid pumping device andsaid fluid working machine are connected using at least one fluidswitching means in particular in a way that the output of said fluidpumping device can be selectively connected to at least one of said atleast two different fluid ports (A, B) of said fluid working machine, inparticular to one of said first (A) and said second fluid port (B) ofsaid fluid working machine.
 4. The fluid flow arrangement according toclaim 1, wherein said fluid flow arrangement is an open hydraulic fluidflow circuit, in particular for propelling purposes.
 5. The fluid flowarrangement according to claim 1, wherein at least one of saidcontrollable fluid throttling devices is a pressure relief valve with apreferably adjustable set point.
 6. The fluid flow arrangement accordingto claim 5, in particular according to claim 5, wherein saidcontrollable fluid throttling device is an electrically controllabledevice and/or in that said controllable fluid throttling device iscontrolled by an electronic controlling device, in particular aprogrammable electronic controlling device.
 7. The fluid flowarrangement according to claim 1, wherein said at least one of saidcontrollable fluid conduit devices is a directional valve, in particulara check valve and/or in that at least one of said controllable fluidconduit devices shows a defined pressure loss behaviour over said fluidconduit device that is dependent on the fluid flow rate through saidfluid conduit device.
 8. The fluid flow arrangement according to claim1, characterized by wherein at least one pressure measuring device, inparticular a plurality of pressure measuring devices that are preferablyarranged at the re-circulating loop, more preferably between a fluidport (A, B) of said fluid working machine and at least one of saidcontrollable fluid throttling devices and/or between at least two ofsaid controllable fluid throttling devices and/or at the fluid outputline of said preferably adjustable fluid pumping device.
 9. Anelectronic controlling device for controlling a fluid flow arrangement,in particular for controlling a fluid flow arrangement according toclaim 1, wherein said fluid flow arrangement comprises at least onefluid working machine, at least a re-circulating loop, fluidlyconnecting a first fluid port (A) and a second fluid port (B) of a fluidworking machine, and at least one controllable fluid throttling devicethat is arranged in said re-circulating loop, characterized in that saidelectronic controlling device generates a control signal for said atleast one controllable fluid throttling device in a way to generate adefined decelerating force for said fluid working machine.
 10. Theelectronic controlling device according to claim 9, wherein at least onesensor signal, describing the current state of the fluid flowarrangement, is used for generating said control signal, in particularin that pressure data is used for generating said control signal. 11.The electronic controlling device according to claim 9, wherein saidcontrol signal is generated in a way that the fluid flow arrangement canbe operated in at least one mode, taken from the group comprising: amethod, in which the speed of the fluid working machine is controlled byoutputting an appropriate control signal to control the pressure at anoutlet port of the fluid working machine, while the fluid workingmachine is not driven by a fluid pumping device; a method, in which thespeed of the fluid working machine is controlled by outputting anappropriate control signal to control the pressure at the outlet port ofthe fluid working machine, while the fluid working machine is driven, atleast in part, by a fluid pumping device; and a method, where theturning direction of the fluid working machine is reversed by firstslowing down the speed of the fluid working machine and then switching afluid switching means in a way that the output of a fluid pumping deviceis selectively connected to a different fluid port of said fluid workingmachine.
 12. A fluid flow arrangement comprising a preferably adjustablefluid pumping device, a fluid working machine fluidly connected to saidfluid pumping device and a re-circulating loop, fluidly connecting afirst fluid port (A) and a second fluid port (B) of said fluid workingmachine, where said first (A) and said second fluid port (B) arepreferably at times at a different pressure level, wherein there-circulating loop comprises a controllable fluid throttling device,wherein an electronic controlling device according to claim
 9. 13. Thefluid flow arrangement according to claim 11 that is used as apropelling means for a vehicle, in particular for a land vehicle. 14.The fluid flow arrangement according to claim 2, wherein said adjustablefluid pumping device and said fluid working machine are connected usingat least one fluid switching means, in particular in a way that theoutput of said fluid pumping device can be selectively connected to atleast one of said at least two different fluid ports (A, B) of saidfluid working machine, in particular to one of said first (A) and saidsecond fluid port (B) of said fluid working machine.
 15. The fluid flowarrangement according to claim 2, wherein said fluid flow arrangement isan open hydraulic fluid flow circuit, in particular for propellingpurposes.
 16. The fluid flow arrangement according to claim 3, whereinsaid fluid flow arrangement is an open hydraulic fluid flow circuit, inparticular for propelling purposes.
 17. The fluid flow arrangementaccording to claim 2, wherein at least one of said controllable fluidthrottling devices is a pressure relief valve with a preferablyadjustable set point.
 18. The fluid flow arrangement according to claim3, wherein at least one of said controllable fluid throttling devices isa pressure relief valve with a preferably adjustable set point.
 19. Thefluid flow arrangement according to claim 4, wherein at least one ofsaid controllable fluid throttling devices is a pressure relief valvewith a preferably adjustable set point.
 20. The fluid flow arrangementaccording to claim 1, wherein said controllable fluid throttling deviceis an electrically controllable device and/or in that said controllablefluid throttling device is controlled by an electronic controllingdevice, in particular a programmable electronic controlling device.